Pain is one of the most feared symptoms for patients with cancer and other serious diseases. It is defined by the International Association for the Study of Pain as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage” (Suri et al., Pharmacokinetic-Pharmacodynamic Relationships for Analgesics, International Journal of Clinical Pharmacology and Therapeutics, 35(8), 307-323, 1997). Pain control is important for patients suffering from cancer, as well as post-surgical patients.